PR #19096: Add F4E2M1FN and F8E8M0FNU types

Imported from GitHub PR openxla/xla#19096

This PR adds F4E2M1FN primitive type (4-bit float with 2 bits exponent and 1 bit mantissa), F8E8M0FNU primitive type (8-bit float with 8 bits exponent, no mantissa and no sign) and enables loads/stores in the same way S4/U4 type is implemented.

This will enable using microscaling (MX) formats ([RFC](openxla/xla#18085)), such as MXFP4.

```c...

PiperOrigin-RevId: 709153611

ml_dtypes/include/mxfloat.h

@@ -339,6 +339,7 @@ namespace Eigen {
 namespace numext {
 
 #define MXFLOAT_EIGEN_SIGNBIT_IMPL(Type)                              \
+  template <>                                                         \
   EIGEN_DEVICE_FUNC EIGEN_ALWAYS_INLINE Type signbit(const Type& x) { \
     int8_t t = bit_cast<int8_t, Type>(x) << (8 - Type::kBits);        \
     return bit_cast<Type, int8_t>(t >> 7);                            \